1. Field of the Invention
The present invention relates to a method for forming a resist pattern. More particularly, it relates to a method for forming a resist pattern which allows reduction of variations in dimensions of the resist pattern which have conventionally occurred depending upon the density of lines in the resist pattern.
2. Description of Related Art
Recent development of micro-fabrication technique in manufacture of semiconductor devices has owed much to photolithography and has been achieved mainly by shortening the wavelength of exposure light. In micro-patterning in dimensions of 0.25 μm or larger, with the wavelength of exposure light used being shortened to g-line (436 nm), i-line (365 nm) and then KrF excimer laser (248 nm), the minimum patterning dimensions have been set equal to or slightly larger than the wavelength of the used exposure light.
However, now, the price of exposure apparatuses has risen sharply with the shortening of the wavelength, and micro-fabrication/ultra-resolution techniques other than wavelength shortening techniques have now been developed, including scanner exposure techniques, modified illumination techniques, ultra-resolution mask techniques and the like. Thereby the minimum patterning dimensions have been further decreased below 0.18 μm, and there has been a phenomenon that the patterning dimensions decrease below the wavelength of exposure light without further shortening of the wavelength. In such circumstances, the above-mentioned various ultra-resolution techniques are fully used in photolithography, but there have occurred some problems which have not taken place in the photolithography of patterning lines in dimensions larger than the wavelength of exposure light used.
Among such problems, the deterioration of MEEF (mask error enhancement factor) is the most serious. MEEF is represented by a numerical value obtained by dividing a change in dimensions of a resist pattern formed on a wafer by a change in dimensions of a mask (one-fold conversion value) and is used as an index representing an amplification ratio of variations in resist pattern dimensions on the wafer with respect to variations in mask dimensions.
For example, referring to FIG. 5(a), in the case of an isolated pattern composed of a line of about 180 nm width, since a space size (the size of a region through which the exposure light passes) is constant and the state of diffracted light is stable (angle of diffraction changes only slightly), MEEF is substantially about 1.0 both in photolithography for patterning in dimensions smaller than the wavelength of the used exposure light (a thin line shown in FIG. 5(a)) and in photolithography for patterning in dimensions larger than the wavelength of the used exposure light (a bold line shown in FIG. 5(a)). On the other hand, in the case where the pattern is not isolated, MEEF increases to 1.5 to 4 depending upon the size, pitch, layout and configuration of the pattern in photolithography for patterning in dimensions smaller than the wavelength of the used exposure light. Accordingly, variations in the dimensions of a mask (one-fold conversion value) are transferred on a resist pattern formed on a wafer with 1.5- to 4-fold amplification. That is, as shown by a bold line in FIG. 5(b), in a pattern composed of repeated lines/spaces of about 180 nm width, since the space size is relatively small with respect to the wavelength, the angle of diffraction changes greatly with respect to a change in the space size. Consequently, MEEF deteriorates to about 2.5.
Also, in proportion with progress of size-reduction, more rigid allowances (specifications) for variations in the mask dimensions are demanded, and since MEEF deteriorates, severer specifications are required to be fulfilled about the mask dimensions.
On the other hand, in the manufacture of masks, some problems have also arisen with regard to achievement of high dimension accuracy. Of such problems, the most acute one is occurrence of differences in dimensions of a resist pattern for forming a mask between dense and sparse regions of the resist pattern owing to a fogging effect. Referring to FIG. 6, the fogging effect means the phenomenon that, of electrons (represented by a) incident onto an EB resist layer 21 formed on a mask substrate 20 from an electron beam source 23 via an EB electrooptic system 22, secondary (reflected) electrons (represented by b) is reflected by the EB electrooptic system 22 and becomes incident again on the EB resist layer 21.
That is, in a pattern 30 having a layout as shown in FIG. 7, a line A around which an exposure region 31 has a large area has smaller resist pattern dimensions than a line B since the amount of re-reflected electrons is large (i.e., the sum total amount of exposure is large) because of the fogging effect. Further, a line C around which the exposure region 31 has a small area has larger resist pattern dimensions than the line B (i.e., resist pattern dimensions become A<B<C).
The fogging effect tends to be intensified depending upon acceleration voltage and exposure amount of an EB exposure system in principle. No currently commercially available EB exposure systems can control the above phenomenon.
With regard to a layout as shown in FIG. 2, for example, if a positive resist is exposed with a dimension-measuring pattern region 10 fixed and an outer dimension W of a surrounding exposure region 11 (W=the entire width—the width of the dimension measuring pattern region 10) increased to 0 mm, 10 mm, 20 mm and 30 mm, a resist space dimension 12 in the dimension-measuring pattern region 10 increases depending upon an increase in the surrounding exposure region 11, that is, an increase in fogging amount. It is not confirmed that this tendency does not plateau even at W=30 mm. This shows that electrons reflected from an extremely large range of the order of several tens mm contribute to the fogging effect.
Additionally, Japanese Unexamined patent Publication Nos. HEI 5(1993)-217875 and HEI 6(1994)-61132, for example, have proposed methods for forming fine patterns with high resolution and high contrast. However, these methods cannot realize fine patterning with higher accuracy by preventing or suppressing the deterioration of MEEF and the influence of the fogging effect in the photolithography for performing patterning dimensions smaller than the wavelength of the used exposure light.